This invention relates to a protecting apparatus for secondary excitation type variable speed AC generator/motors and more particularly to a protecting apparatus adapted for a variable speed AC generator/motor connected to an electric power system and having a secondary winding which is AC excited by means of cycloconverters.
With regard to power controlling in a variable speed AC generator/motor such as an induction generator/motor connected to an electric power system, it has been known to control effective power and reactive power fed to and received from the electric power system by controlling secondary excitation for the motor, as disclosed in, for example, Japanese Patent Publication (Kokoku) No. 53-7628 published on Mar. 20, 1978 and No. 57-60645 published on Dec. 21, 1982. When this type of variable speed AC generator/motor is increased in capacity so as to be used as a large-capacity generator/motor in a pumping-up power station, the power line voltage is used to control secondary excitation currents by way of power converter units such as cyclo-converters.
In the event that a voltage variation occurs, due to an accident in the electric power system and/or an ON/OFF operation of an AC breaker associated with another generator/motor, during controlling of the secondary current of the generator/motor, a DC transient component of primary current induces a rotating frequency component which is superimposed on a slip frequency current component on the secondary side. As a result, a current larger than the normal secondary current flows through a cyclo-converter. In such an event, if the value of this current can be suppressed to not larger than a rated range of the device such as the cyclo-converter, it is desirable from the standpoint of stabilization of the overall system that the power generation or the pumping-up operation is continued by separating out the parts which failed. To realize this operation, the withstanding ability of the device may conceivably increased but such an expedient is not only economically disadvantageous but also impossible to implement in many technical points if employed for a pumping-up generation apparatus of large capacity. Therefore, an overcurrent flowing through the cyclo-converter needs to be suppressed. But since the current suppressing control has to be done before the overcurrent reaches an overcurrent level of thyristors, detection of the accident in the electric power system must be discerned instantaneously.
On the other hand, an overcurrent will likewise flow for causes such as an internal accident of the cyclo-converter and/or a failure to commutate by the cyclo-converter. In such an event, the operation must be ceased instantaneously and controlling for suppressing damage of the master machine to a minimum must be undertaken. However, because of only detection of the overcurrent itself, it has hitherto been impossible to discern different causes for the occurrence of the overcurrent. It follows therefore that the conventional controlling is undertaken to simply stop the main machine upon occurrence of any type of accident wth a view of protecting the main machine, raising a problem that when it is applied to a variable speed pumping-up/power generation apparatus of large capacity connected to the electric power system, stability of the electric power system is degraded.
In the past, an overvoltage protector for a power converter unit of the type as disclosed in, for example, Japanese Patent Application (KOKAI) JP-A-58-12530 has been known. In this reference literature, when DC current is supplied to a DC load from the AC power supply via a power converter unit comprised of thyristor elements, the power converter unit can be protected against an overvoltage due to interference of a surge or the like stemming from the AC power supply by closing protecting thyristor switches connected in parallel with the thyristor elements in response to the application of the overvoltage across the thyristor elements. But, this prior art is far from protecting the power converter unit against an overvoltage induced in its secondary winding when an accident takes place in the electric power system to which the secondary excitation type AC generator/motor is connected and has of course nothing to do with the problem involved in continuity and stability of the operation of the electric power system. Another reference literature, Japanese Patent Application (KOKAI) JP-A-56-166796, may be referred to as disclosing a protector for a thyristor Scherbius device connected to the secondary winding of a secondary excitation type induction motor. Fundamentally, when an overvoltage occurs while the motor is running at speeds below the synchronous speed, the protector detects the overvoltage and causes thyristor switches to short-circuit the secondary winding, thereby protecting thyristor elements. Especially, in the latter prior art, a plurality of thyristor switches are connected in parallel and selected ones of the thyristor switches, which are selected in accordance with the rotating speed of the motor, are closed to minimize variations in torque of the motor. The latter prior art, however, fails to take account of the variable speed AC generator/motor having an operation range in which the secondary excitation is controlled so that it runs at speeds above the synchronous speed to serve as a generator.
Any of the prior art overvoltage protectors described above intend to protect the thyristor elements and supplementarily ensure stable operation of the motor and never teach a protecting apparatus operative to protect thyristor elements of each power converter that connected in the secondary excitation circuit for the variable speed AC generator/motor connected to the electric power system while taking account of continuity of the operation necessary for sustaining stability of the electric power system.
It is of significance to understand that when an accident occurs in the electric power system to which the variable speed AC generator/motor is connected, overvoltages do not appear concurrently at the three phases but an overvoltage is caused separately in each phase. Accordingly, if the secondary winding is short-circuited instantaneously in response to the occurrence of the overvoltage, the variable speed AC generator/motor will instantaneously shift to the motor operation when it is running at speeds below the synchronous speed to serve as a generator and will instantaneously shift to the generator operation when running at speeds above the synchronous speed to serve as a motor, thus inviting an increase in variation of torque and making continuity of the operation difficult.